(a) Field of the Invention
The present invention concerns an apparatus for performing positron emission computed tomography, and more particularly it pertains to an improvement of the arrangement of the detectors employed in this apparatus.
(b) Description of the Prior Art
There have been proposed apparatuses for effecting a measuring, externally of the subject to be examined, of intra-body distribution of positron-emitting radioisotopes which are administered into the body of the subject, and those apparatuses designed for displaying images of a slice on a cathode ray tube.
FIG. 1 schematically shows the principle of such known apparatus. In FIG. 1, reference numeral 1 represents a subject for examination. 2 represents respecrive gamma (.gamma.) ray detectors (hereinafter to be referred to briefly as detectors). 3 represents an array of detectors which is constructed by a row of a plurality of detectors. 4 represents a coincidence counting circuit. 5 represents a data collecting and recording means. 6 represents a data processing means. 7 represents an image display means. The detectors included in the array 3 of detectors are disposed on a plane containing a slice of the subject 1 for examination which is to be displayed, and these detectors are assigned to detect gamma rays due to the isotopes contained in the body of the subject 1.
The isotope which is administered into the body of subject 1 is selected from those substances, such as carbon 11 (.sup.11 C), nitrogen 13 (.sup.13 N) and fluorine 18 (.sup.18 F) which emit positrons. Positrons which are emitted from atomic nuclei of the isotopes cause annihilation reaction with the electrons which are present at sites very close to those positrons emitted. As a result, there are generated two annihilation gamma photons which are emitted in opposite directions (in the directions of 180.degree.) at the same time. Let us now suppose that two of the detectors 2, each representing one in the respective arrays 3 of detectors which are provided on both sides of the subject 1 in FIG. 1 have detected annihilation gamma photons at the same time. Then, the positions at which these annihilation gamma ray photons are generated, i.e. approximately the positions of the isotope atoms which have emitted positrons, are considered to be on the straight line connecting these two detectors. It should be understood that each of those solid lines connecting two detectors which are contained in the respective arrays 3 of detectors represents an imaginary straight line passing through the position of the isotope atom which is found by detecting the coincidence count performed by the two detectors connected by that straight line. The abovesaid imaginery straight line indicating the position of the isotope atom which is found out can be designated by the positional coordinate of the opposing two detectors which participate in the coincidence counting. It should be understood, however, that such position of the isotope atom which is found out may be designated also by the two factors, i.e. the distance t from the origin of an appropriate coordinate system which is fixed in the subject 1 and which lies in a plane containing the arrays 3 of detectors to the aforesaid imaginary rectilinear line and which has the coordinate origin at the center of rotation, and the angle .theta. defined by this imaginary rectilinear line relative to an axis of coordinate. In this specification, the position of the isotope atom which is detected is represented by a rectilinear line connecting two detectors, and it is expressed by the distance t and the angle .theta., and such position is called a sample position or sampling position, and the aforesaid two detectors which are electrically connected by a coincidence counting circuit for detecting a coincidence count are called a detecror pair. The emission of positron which is thus detected by an appropriate detector pair connected together by a coincidence counting circuit is recorded of its number of count and sampling position (t, .theta.), by a data-collecting-and-recording means 5. Those data which thus have been recorded within a certain period of time are then subjected to rearrangement and reconstruction of the data at a data processing means 6, whereby an image of distribution of isotopes in that portion of the body of subject which is sliced at said plane, i.e. a cross-sectional image, is synthesized, and it is displayed by an image indicating means 7.
FIG. 2 shows an arrangement of detectors in a known scanner for positron emission computed tomography which is based on the foregoing principle. In FIG. 2, there are depicted thirty six (36) detectors to facilitate understanding. It should be noted, however, that in practice there are arranged 60.about.200 detectors. It this known apparatus, detectors 2 are arranged with equal spacing on a circular circumference surrounding the subject 1, and these detectors are connected to coincidence counting circuits for detecting coincidence counts between them and those detectors which are located on the other side relative to the center C of the circular circumference. It should be understood that a certain detector located on this side is not coupled to only one detector located on the other side to make a pair, but to a plurality of detectors via a coincidence counting circuit. All of those detectors which are located on the other side relative to a certain designated detector located on this side, and which are located within the range of coverage by this certain detector are coupled to this certain detector by coincidence counting circuits, respectively. For example, the certain designated detector is coupled, by coincidence counting circuits, to all of those detectors which lie within an angle .+-.30.degree. as viewed from this certain detector relative to the rectillinear line connecting this certain detector and the center C of the aforesaid circular circumference.
A scanner of the known apparatus having such arrangement of detectors as stated above is capable of making detection as well as coincidence counting of those gamma ray pairs which are emitted at arbitrary positions lying within a circle of a radius which is about 1/2 of the radius of the circular circumference on which the detectors are arranged, without requiring mechanical movement such as rotation of the arrays of detectors, and thus the scanner is able to determine their sampling positions (t, .theta.).
In such known apparatus, the values of t which are obtained represent discontinuous descrete values having distance intervals substantially equal to the spacing of the detectors. On the other hand, the values of .theta. obtained are only discontinuous discrete values with intervals representing an angle defined by a detector and adjacent two detectors which are located on the other side relative to the center C. Apart from the above, the smaller the respective intervals of the values t and of the values .theta. are, the higher can be improved the quality of the image which is reconstructed. Thus, with such known apparatus, it is not possible to obtain intervals of t or .theta. which are sufficient for obtaining a quality image. Also, if a large number of detectors are arranged to make the intervals of .theta. or of t sufficiently small, it will be obvious that the cost of manufacture will be increased markedly.
On the other hand, there has been proposed to improve the quality of image, in this known apparatus, by first taking a measurement once at a stationary state of the apparatus, and thereafter repreating the measurement after revolving the arrays of detectors through an angle which is 1/2 of the angle defined by the center C and adjacent two detectors located on one side relative to this center C, thereby making the respective intervals of t and of .theta.1/2 relative to the intervals obtained at the time of measurement at stationary state of the arrays of detectors. It should be understood, however, that even from further continuation of such revolution, it is not possible to reduce the intervals of t and of .theta. any further. Furthermore, even when a measurement is taken by making the angle of one revolution sufficiently small, there will arise no change in the intervals of t which control the resolution of image, though the intervals of .theta. will become reduced accordingly.
Description will hereunder be made of the manner of expressing the fineness of the sample positions, in order to facilitate the understanding of the present invention.
In FIG. 3, point C and the rectilinear line XX represent the origin and the coordinate axis of the coordinate system which is fixed to the body of the subject for examination. The array of gamma ray detectors is rotated about this point C. Point i and point j represent appropriate gamma ray detectors which make a pair among the array of detectors. r.sub.i and r.sub.j represent the distances from the center C of rotation of the detectors i and j. a.sub.i and a.sub.j represent angles defined by r.sub.i and r.sub.j relative to the axis of coordinate axis XX. t.sub.ij and .theta..sub.ij represent the distance from point C to the rectilinear line connecting the detectors i and j, and the angle formed by this rectilinear line relative to the coordinate axis XX, i.e. the sampling position of the information obtained by the coincidence counting by the detector pair i and j, respectively. Let us now assume that, with respect to all angles, those angles going counter-clockwise are designated as positive angles, and that those going clockwise are designated as negative angles. Then, the sampling positions .theta..sub.ij and t.sub.ij of an arbitrary detector and obtained, based on FIG. 5, by the below mentioned formulas: ##EQU1##
It is to be noted here that the chart in which .theta..sub.ij and t.sub.ij in Formulas (1) and (2) are expressed in a single chart for all the detectors will hereunder be called the t, .theta. distribution chart.
FIG. 4 is a chart of distribution of t and .theta., in the instance wherein the known apparatus shown in FIG. 2 is not rotated. In FIG. 4, the vertical axis represents distance t, and the horizontal axis represents angle .theta., and the respective sampling positions (t, .theta.) are shown by small circles. These Figures are provided so that, for the convenience of calculation, the spacing between the detectors is selected at 2 cm and the radius of this aforesaid circle is selected at about 11.5 cm. Also, the visual angle of each detector is set so as to be .+-.30.degree.. As will benoted from FIG. 7, in the known apparatus, the interval of t at sampling points where .theta. is identical, in case the detectors are not rotated, is equal to the spacing of 2 cm between the detectors in the central portion, whereas in the peripheral portion, i.e. in the region where t is large, the interval of t will become slightly smaller than the spacing between the detectors. Also, in case the array of detectors is rotated through an angle of 5.degree. relative to the center C of the circle, i.e. in case the array of detectors is rotated through an angle which is 1/2 of the spacing between the detectors, and in case data are collected before and after such rotation, the sampling points of the data in this latter case will be exhibited as those obtained by translating or moving the distribution of the former by 5.degree. for the horizontal axis. Accordingly, as will be noted from the Figure, the interval of t when .theta. is identical will become 1/2 of that obtained when the detectors are not rotated. However, even when a further rotation is made, the result is that sampling points are merely superposed, and it is not possible to reduce the interval of t.
The known apparatus which is called PETT 1 (H. J., De Blanc and J. A. Sorenson (ed.); Noninvasive Brain Imaging: Computed Tohography and Radionuclides, the Society of Nuclear Medicine, Inc., New York, 1975 pp 87-109) adopts the method such that, as shown in FIG. 5, four detectors are disposed at an equal interval on each side of a regular hexangle cocentral with the center of rotation, so that coincidence counting is carried out between those detectors located on the opposing two sides, for example, a-a' of the regular hexangle. Furthermore, in this known apparatus, the respective lines, O.sub.1 -O'.sub.1, O.sub.2 -O'.sub.2 and O.sub.3 -O'.sub.3 connecting the centers of the respective arrays of opposing two sides of the hexangle are arranged to be at different distances, respectively, from the center of rotation, so as to be operative that, by rotating the detectors through 360.degree., those values of t of the sample positions which are produced by the detector pairs a-a', b-b' and c-c' are different relative to each other. In this known apparatus, however, those detectors in the respective arrays are disposed at an equal interval, and accordingly, when a single detector is considered, conincidence counting is carried out between it and only those detectors which are disposed at an equal interval, so that no sufficient distribution of the values of t of the sample position is obtained.